1. Field of the Invention
The present invention relates to an optical system for an automatic lens meter to measure the refractive power of a lens, and more particularly to an optical system for an automatic lens meter to measure the refractive power of a lens by arranging a concave lens for expanding a light beam downstream of the lens to be examined so that the expanded light beams pass through apertures and then clear images are formed on a sensor array, whereby it is easy to manufacture optical parts and convenient to assemble the optical system for an automatic lens meter.
2. Description of the Related Art
As well known to those skilled in the art, a conventional optical system used in an automatic lens meter to measure the refractive power (diopter) of a lens, as shown in FIGS. 1 and 2, includes a light source 1 for generating an examining light beam for measuring the refractive power of a lens, a collimator lens 2, disposed upstream of the lens to be examined, for converting the generated light beam into parallel light beams to enter the lens to be examined, a plural-apertures stop 4, formed with several apertures 4a (usually, 4 to 6 apertures) and disposed downstream of the lens 3 to be examined, for separating the parallel light beams, focused or diverged depending on the refractive power of the lens 3 to be examined and transmitted through the lens 3 to be examined, at a predetermined angle, a prism 5, disposed downstream of the plural-apertures stop 4, for rotating and deflecting the light beams transmitted through the plural-apertures stop 4, a convex lens 6 for focusing the light beams transmitted through the prism 5 on a sensor array 8, and a cylindrical concave lens 7 for expanding the light beams transmitted through the convex lens 6 in a direction perpendicular to the direction of arrangement of the sensor array 8.
In a refractive power measuring apparatus including the conventional optical system constructed as described above, an examiner fixes the lens 3 to a lens holder disposed upstream of the plural-apertures stop 4. The examiner measures the refractive power of the lens 3 by adjusting the optical center of the lens 3 to align it on the center of a computer screen, that is, an optic axis of the lens meter by moving the lens 3 while viewing the computer screen.
The light beam emitted from the light source is converted into parallel light beams, by the collimator lens 2, which then enter the lens 3. The parallel light beams pass through the lens 3, are focused or diverged depending upon the refractive power of the lens 3, so as to be separated into a plurality of light beams equal to the number of the apertures 4a, by passing through the plural apertures 4a formed at the plural-apertures stop 4.
The light beam for measuring is separated by the apertures separately penetrating the plural-apertures stop 4 at predetermined angular intervals, while passing through the plural-apertures stop 4, and passes through the prism 5. The separated light beams for measuring are rotated and deflected in the normal direction of a one-dimensional sensor array 8. The light beams pass through the convex lens 6 for focusing images and the cylindrical concave lens 7 disposed at the rear of the convex lens 6, sequentially. The light beams for measuring pass through the cylindrical concave lens 7 and form images on the sensor array 8. The examiner viewing the computer screen measures the refractive power of the lens 3 by aligning the central axes of the images formed on the sensor array 8 with the optic axes of the lens meter.
For the purpose of arranging images formed by the six light beams, having passed through the plural-apertures stop 4 and separated every 60 degrees, on the one-dimensional sensor array 8, the conventional optical system for the automatic lens meter, constructed as described above, uses the rotating function of the prism 5 to rotate the light beams, deviated from the normal direction of the sensor array 8, to be aligned with the normal direction of the sensor array 8. The conventional optical system for the automatic lens meter uses the deflecting function of the prism 5 and the cylindrical concave lens 7 in order to maintain uniform distances between the images arranged on the sensor array 8.
In order to manufacture the prism 5 for rotating and deflecting the light beams, since the same number of triangle-shaped prisms as the number of apertures must be arranged at each location so that they are aligned with each aperture, and since respective incident surfaces and light-emitting surfaces of the triangle-shaped prisms must be kept at a predetermined angle, the structure of the optical system is complex and it is difficult to manufacture components of the optical system.
Further, since, the cylindrical concave lens 7 must be used in order that the light beams for measuring reach the sensor array 8 even if the light beams for measuring travel in the tangential direction perpendicular to the normal direction of the sensor array 8, and since the optic axes of the light beams for measuring must constantly be aligned with the normal direction of the sensor array 8, operational efficiency deteriorates due to the long time required to assemble the optical system, and manufacturing costs are high due to the large number of parts.
More particularly, since very small apertures must be formed in order that the light beams pass through the plural-apertures stop 4 and form precise images of the apertures on the sensor array 8, the cost to manufacture the plural-apertures stop 4 increases. In the event that foreign matter is attached to the very small apertures 4a, since the images are not formed on the sensor array 8, or since the image signal may be seriously inferior, it is difficult to precisely measure the refractive power of the lens 3 to be examined.